In the case of preparing a fluorine-containing polymer by an emulsion polymerization method, a method of carrying out emulsion polymerization by using an emulsifying agent to stabilize a polymerization system and a method of carrying out emulsion polymerization by decreasing an amount of an emulsifying agent to a great extent or without using an emulsifying agent have been known for long.
The latter method is a method of carrying out emulsion polymerization without adding an emulsifying agent by utilizing conversion where a water soluble polymerization initiator such as ammonium persulfate (APS) used for emulsion polymerization acts so as to form an end group of the obtained fluorine-containing polymer into an ionic end group such as carboxyl (JP48-18957B).
In this emulsion polymerization method, a large amount of a water soluble polymerization initiator is added initially to produce, at an initial stage of the polymerization, a polymer having ionic functional group at its end at high concentration, and this polymer having ionic functional group at its end has an emulsifying action and functions, as a so-called polymer emulsifying agent, to attain an emulsified state of a polymerization system, namely, micelles are formed. Thereafter, as the polymerization proceeds, these micelles grow to become emulsified particles which further grow into larger particles. Such polymer emulsifying agent particles are produced one after another during proceeding of the polymerization, and therefore, even if growing of particles of polymer emulsifying agent proceeds at an initial stage of polymerization and an emulsifying action is lowered, the polymerization proceeds under stable emulsified state. This indicates that for carrying out emulsion polymerization stably, particles of polymer emulsifying agent need be newly produced as the polymerization proceeds. As a result, an obtained fluorine-containing polymer is obliged to become a fluorine-containing polymer having a lot of ionic ends.
Also, methods of carrying out emulsion polymerization of a fluorine-containing monomer by two steps are known (JP52-84271A and WO 96/17876). These polymerization methods do not aim at producing particles of polymer emulsifying agent, and an object thereof is to control the number of seed particles in a polymerization system before initiating desired emulsion polymerization (second step), or to stably disperse an oil soluble radical polymerization initiator to be used in the following step.
For example, in JP52-84271A, an elastic polymer at least containing vinylidene fluoride (VdF) is produced by two steps. In the first step, VdF and other fluorine-containing monomer are subjected to emulsion polymerization in the presence of a water soluble radical polymerization initiator, and after decomposition of the water soluble radical polymerization initiator is carried out if necessary, in the second step, emulsion polymerization is carried out using an oil soluble radical polymerization initiator. JP52-84271A discloses that in the emulsion polymerization of the first step, an obtained fluorine-containing copolymer functions as a dispersion stabilizer due to an action of hydrophilic group derived from the water soluble radical polymerization initiator, and therefore, no emulsifying agent (dispersion stabilizer) need be used in not only the first step but also the second step.
JP52-84271A says that an amount of the water soluble radical polymerization initiator used in the first step is 0.01 to 20% by mass, preferably 0.05 to 10% by mass based on the monomer used in the first step, but an amount thereof used in examples is at most up to 1.5% by mass based on the VdF copolymer to be obtained in the first step. In addition, though it is said that an amount of a VdF copolymer to be prepared in the first step is 1 to 80% by mass based on the VdF copolymer to be finally prepared, in examples, the amount is as high as not less than 5% by mass based on the finally obtained VdF copolymer.
Even under the conditions described in examples of JP52-84271A, emulsion polymerization of the first step and the second step can be surely carried out without using an emulsifying agent (dispersion stabilizer), and an aimed VdF copolymer can be prepared. However, a produced polymer is easily adhered on a wall surface of a polymerization reactor during polymerization, and use of an emulsifying agent cannot be avoided in order to solve this problem.
Also, WO 96/17876 indicates that in a two-staged emulsion polymerization method as described in JP52-84271A, by further decreasing an amount of a water soluble radical polymerization initiator used in the first step, an elastic fluorine-containing copolymer having well-balanced characteristics such as good flowability, vulcanization rate and physical properties of rubber is obtained.
In WO 96/17876, an amount of water soluble radical polymerization initiator used in the first step is so controlled as to be 0.001 to 0.003% by mass based on the total produced copolymer, and in example thereof, in the first step, in order to produce a fluorine-containing copolymer in an amount of 3 to 10% by mass based on the total produced copolymer, 0.0016 to 0.0022% by mass of water soluble radical polymerization initiator based on the fluorine-containing copolymer to be prepared in the first step is used. Though the amount of fluorine-containing copolymer to be prepared in the first step is decreased, an amount of water soluble radical polymerization initiator is greatly decreased, and a problem pointed out in JP52-84271A such as adhesion on a wall surface of a polymerization reactor is not solved.
In addition, JP2006-504844A discloses emulsion polymerization for preparing a fluorine-containing polymer in two steps without using an emulsifying agent. In the first step, emulsion polymerization of VdF with other fluorine-containing monomer or non-fluorine-containing monomer is carried out in the presence of a large amount of water soluble radical initiator to form fluoropolymer particles (field of polymerization) and prepare an emulsion. In the following second step, polymerization for preparing a fluorine-containing polymer is carried out successively without treatment of this emulsion. Since water soluble radical initiator is used in the first and second steps in an amount of not less than 1% by mass based on the finally obtained fluorine-containing copolymer, as mentioned above, the obtained fluorine-containing copolymer is obliged to have a lot of ionic ends. In addition, in JP2006-504844A, there is no description with respect to a problem with adhesion of produced polymer on a wall surface of a polymerization reactor as pointed out in JP52-84271A.
On the other hand, a so-called iodine transfer polymerization method, in which emulsion polymerization is carried out in the presence of an iodine compound, is known as a process for preparing a fluorine-containing rubber (JP53-125491A and WO 00/01741). In this iodine transfer polymerization method, generally emulsion polymerization is conducted using a small amount of water soluble radical polymerization initiator (for example, APS) in the presence of a fluorine-containing emulsifying agent comprising a perfluoro chain having about 4 to about 10 carbon atoms. An end of the fluorine-containing rubber obtained by an iodine transfer polymerization method is terminated with an iodine-containing group at high probability, and as a result, vulcanization characteristics and strength and compression set of a crosslinked article are improved. The reason for decreasing an amount of APS is that if the number of ionic functional groups (carboxyl groups) derived from APS is increased on ends of the obtained fluorine-containing rubber, an amount of end groups terminated with iodine-containing groups is decreased, and as a result, the above-mentioned excellent effects cannot be obtained. For the purpose of prevention of it, use of an emulsifying agent is necessary.
Accordingly, in an iodine transfer polymerization method, as described in JP48-18957B and JP2006-504844A, if a method of preparing a polymer emulsifying agent in situ is employed for decreasing an amount of an emulsifying agent used, since the number of carboxyl groups are increased as end groups of the obtained fluorine-containing rubber (a ratio of termination with iodine atoms is decreased), advantages of an iodine transfer polymerization method cannot be obtained.
As mentioned above, since it is important that an amount of ionic functional groups is not increased in an iodine transfer polymerization method, there have been no ideas of applying such a two-staged emulsion polymerization method as disclosed in JP52-84271A and WO 96/17876, in which many ionic functional groups are generated in the first step without using an emulsifying agent.